1. Field of the Invention
The present invention relates to a dose rate measuring apparatus introduced to an environmental radiation monitoring post or a portable monitoring post set up within a nuclear-related facility and in a surrounding area to measure a dose rate that is an amount of radiation per unit time.
2. Background Art
For a dose rate measuring apparatus in the related art, there are a G(E) function method and a DBM (Discrimination Biased Modulation) method as means for finding a dose rate or the like on the basis of a wave height of a detection pulse. According to the G(E) function method, an energy spectrum of radiation is obtained by an MCA (Multi-Channel Analyzer) and a dose rate is obtained by multiplying energy conversion factors (hereinafter, referred to as the G(E) functions) that are energy conversion factors corresponding to energy of respective channels of the obtained spectrum by the numbers of counts of the respective channels. According to the DBM method, a wave height discriminator that discriminates a wave height of a detection pulse is used and a probability that the detection pulse is inputted into a counter at a latter stage in the wave height discriminator is adjusted according to the wave height by temporally varying a discrimination threshold of the wave height discriminator in accordance with a predetermined pattern.
When a dose rate rises, a transmittance is found and a variance of the transmittance is used as information on the basis of which to determine whether a rise in dose rate is contributed by natural radon and thoron accompanying rainfall or snowfall or contributed by a reactor facility. A dose rate is found from a transmittance as follows. First, a transmittance is found by the G(E) function method by converting a mean dose rate per unit time, for example, one minute, to a count rate of γ rays of energy equivalent to 3 MeV and by dividing this count rate by a count rate of all the γ rays in a measurement range. Alternatively, a transmittance is found by dividing a count rate obtained by the DBM method by a count rate of all the γ rays in a measurement range. The transmittance thus found is subjected to processing in the MCA while maintaining a measurement time constant to obtain an energy spectrum of radiation. A dose rate is obtained by multiplying a conversion factor between a dose rate and energy corresponding to the energy of the spectrum by the numbers of counts of the respective channels of the MCA. For details, see Patent Document 1.    Patent Document 1: JP-A-2009-175042 (pp. 3 to 12, FIG. 4)
As has been described, according to the dose measuring apparatus in the related art, a dose rate is outputted together with a transmittance and the transmittance is used as information based on which to determine a cause of an indicated rise. However, there is a problem that a transmittance is indirect and difficult to understand in comparison with mean energy. Also, the G(E) function method and the DBM method in the related art have a problem in responsivity to an abrupt development of an event. Even when an improvement is made in the DBM method by adding an output that quickly responses to a rise in dose rate by holding a statistical error constant, information on a transmittance on the same time axis when the dose rate is measured is absent. Hence, a lack of information in an abrupt development of an event becomes a problem. Further, in a case where energy of radiation is low, a skipping ratio becomes higher in the DBM method. This poses a problem that resolution of a dose rate becomes rough or a count loss occurs when low-energy radiation comes in a burst.